1. Field of the Invention
The present invention relates to a memory control unit that controls the reading and writing of data in a semiconductor memory, and to a memory system including the memory control unit and the semiconductor memory.
2. Description of the Related Art
Many electronic devices include an embedded microelectronic computing device comprising a microprocessor unit and a memory control unit or memory system. The memory system may include a general-purpose memory controller residing in a large-scale semiconductor integrated circuit chip (hereinafter, microchip), to which external semiconductor memory such as read-only memory (ROM) or random-access memory (RAM), or both, is connected; alternatively, the memory controller, ROM, and RAM may all be part of the same microchip.
FIG. 11 illustrates the general structure of a conventional memory of the type in which the memory controller 20 resides in one chip and the external memory 21 in one or more other chips. Access to the external memory 21 is enabled by an enable signal (a chip select signal CS) output from the memory controller 20. Read access or write access is selected by a read/write control signal output by the memory controller 20. In write access, the external memory 21 stores data transferred from the memory controller 20 at a location designated by an eight-bit address (addr) output by the memory controller 20. In read access, the external memory 21 transfers the data currently stored at the designated address to the memory controller 20.
One function of the memory controller 20 is thus to use the enable signal, the read/write signal, and the address signals to control the operation of the external memory 21. Another function is to send data to the external memory 21 and receive data from the external memory 21. Yet another function that may be carried out by the memory controller 20 is to protect the security of the data stored in the external memory 21 by shuffling the address signals, as described in Japanese Patent Application Publication No. H11-175404.
A weakness of this conventional memory system is that since the external memory 21 is externally connected to the memory controller 20, a third party can easily copy the entire contents of the external memory 21 into another external memory. If the memory controller 20 is part of a commercially available general-purpose microchip, the third party can then obtain an identical microchip and connect it to the unauthorized copy of the external memory to duplicate the entire memory system.
One way to thwart this type of unwanted duplication is to customize the microchip that includes the memory controller 20 by adding special features such that software stored in the external memory 21 can be executed by the customized microchip, but not by commercially available general-purpose microchips. This scheme is expensive, however, because the modifications required for customization are complex.
Somewhat similar problems arise when the memory and memory controller are on the same chip. To protect the data in the memory, the memory controller must be provided with special features that make the chip more expensive to manufacture.